Method for making a fiber reinforced composite rivet having an upset head

ABSTRACT

A method for making a composite rivet and upsetting the composite rivet after the rivet has been inserted through (e.g., composite) structural members to be held together. The rivet includes a core having continuous and unidirectionally extending fibers that are reinforced by a thermoplastic resin. The core is surrounded by a braided jacket having fibers arranged in a criss-cross weave. A forming die guide having a containment opening extending therethrough is positioned so that the upstanding end of the composite rivet is received within the containment opening. A heated forming die having a forming cavity is moved into the containment opening of the forming die guide so as to lie in axial alignment with the upstanding end of the rivet. A ram forces the heated forming die through the containment opening so that the upstanding end of the rivet is softened and upset within the forming cavity. The ability of the upstanding end of the rivet to spread out under the heat and pressure generated by the heated forming die is limited by the containment opening of the forming die guide.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a fiber reinforced composite rivet and to amethod for upsetting one end thereof by means of compact, easy to useupsetting tools.

2. Background Art

Plastic and metal rivets are well known fasteners for connectingopposing structural members to one another. However, in certainapplications, particularly those related to the aerospace industry, theweight associated with the conventional rivet can not be ignored. Forexample, when a very large number of rivets is used in an aircraft, thetotal weight of the aircraft is typically increased and the efficiencyof operation is typically reduced.

To overcome the aforementioned weight problem and to provide a reliable,high strength means for connecting together opposing structural members,rivets made from a composite material have been proposed. In this case,a free upstanding end of the composite rivet must be upset during theassembly process in the field. Unfortunately, no compact, easy to usetool is known by which to enable a workman at a job site to upset theupstanding end of a composite rivet after the rivet has first beeninserted through the structural members to be connected together.

SUMMARY OF THE INVENTION

In general terms, a fiber reinforced composite rivet is disclosed thatis capable of being upset so as to reliably connect together opposing(e.g., composite) structural members once the rivet has been insertedthrough the members. A rivet preform is initially positioned in aninsert that is held by a mold base of a force generating press. Therivet preform is formed by continuous (e.g., carbon, quartz, glass,etc.) fibers that run unidirectionally (i.e., longitudinally) throughthe preform. The fibers are reinforced by a thermoplastic (e.g., PEEK orPPS) resin. The rivet preform is surrounded by an outer fiber braidedjacket comprising continuous fibers that are arranged in a criss-crossweave. The fibers of the braided jacket are also reinforced by athermoplastic resin.

A first end of the fiber preform projects upwardly into a female cavityof the insert within which the preform is positioned. The rivet preformand insert are heated in an oven, and the mold base is preheated withinthe press. The heated preform and insert are removed from the oven andlocated in the preheated mold base. The press is closed to applypressure to the first end of the heated fiber preform, whereby the firstend is softened and shaped by the female cavity of the insert so as toestablish a composite rivet having a (e.g., flat) head. Once the preformhas cooled down, the press is opened and the insert is removed from themold base and cooled in water. The composite rivet is then pushed out ofthe insert and deflashed.

The composite rivet is now inserted through the opposing structuralmembers to be connected together such that the newly formed head of therivet lies at one side of the members and the upstanding core of therivet projects to the other side of the members. A forming die guide ispositioned so that the upstanding core of the composite rivet isreceived within a containment opening that is formed in the forming dieguide. A heated forming die having a forming cavity at one end thereofis moved into the containment opening of the forming die guide so as tobe axially aligned with the upstanding core. A ram is coupled to theheated forming die located within the containment opening of the formingdie guide. The ram generates a pressure to cause the heated forming dieto move towards and into contact with the upstanding core of thecomposite rivet, whereby the core is softened and shaped (i.e., upset)by the forming cavity of the forming die. The ability of the upset headto spread out during formation is restricted by the containment openingof the forming die guide which surrounds the upstanding core. A pointedtip within the forming cavity of the forming die leaves a depression inthe upset head which directs the unidirectional fibers to the peripheryof the upset head in order to improve the ability of the composite rivetto withstand tensile loads.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a braided fiber preform having the preferred continuous,unidirectional fiber orientation prior to manufacture of the compositerivet of this invention;

FIG. 2 shows the fiber preform of FIG. 1 located within a heated insertso as to receive a force generated by a press during formation of thecomposite rivet;

FIG. 3 shows the composite rivet after it has been formed and removedfrom the insert of FIG. 2;

FIG. 4A is an exploded view illustrating the upsetting tools by whichthe upstanding core of the composite rivet of FIG. 3 is upset followinginsertion of the rivet through opposing structural members to beconnected together;

FIG. 4B shows the upsetting tools of FIG. 4A coupled to one another andwith the upstanding core of the composite rivet;

FIG. 4C shows the upsetting tools applying pressure to the upstandingcore to provide the composite rivet with an upset head;

FIG. 4D shows the upset head of the composite rivet after the upsettingtools of FIG. 4C have been removed; and

FIG. 5 illustrates the fiber orientation of the composite rivet of FIG.4D having an upset head.

DETAILED DESCRIPTION

FIG. 1 of the drawings shows a rivet preform 1 which will be subject toheat, compression and solidification in order to form a fiber reinforcedcomposite rivet 30 (of FIG. 3) that can be upset (as shown in FIG. 4) bymeans of compact and easy to use tools so that a pair of opposing (e.g.,planar) composite structures can be reliably connected together. Therivet preform 1 (e.g., a pulltruded rod segment) includes a plurality ofresin impregnated fibers 3. For maximum strength and reliability, thefibers running through the rivet preform 1 are both continuous andunidirectional (i.e., longitudinal) rather than chopped or random. Byway of example, the fibers 3 of the rivet preform 1 are manufacturedfrom carbon, quartz, glass, or the like. The fibers 3 are reinforced bya suitable thermoplastic resin such as, for example, that known as PEEK,PPS, or the like. A thermoplastic resin is preferable so as to enableone end of a composite rivet to be upset in a manner that will beexplained in greater detail hereinafter when referring to FIG. 4.

The rivet preform 1 is surrounded by an outer braided jacket 5. Thebraided jacket 5 may be applied over and fused to the preform 1 by meansof a conventional braiding machine. For purposes of efficiency, theapplication and fusing of the braided jacket 5 to the preform 1 may becompleted during a single step. Like the rivet preform 1, the braidedjacket 5 includes a plurality of continuous fibers 7 that are reinforcedby a suitable thermoplastic resin. The braided jacket 5 surrounds therivet preform 1 in a criss-cross weave as shown in FIG. 1.

FIG. 2 of the drawings shows the braided composite rivet preform 1 ofFIG. 1 being retained within an insert 10 of the kind that is typicallyinstalled in a mold base 12. The mold base 12 is preferably manufacturedfrom aluminum to facilitate cooling during the manufacture of the headedrivet 30 shown in FIG. 3. In general, an elongated braided rod (notshown) is first produced (i.e., pulltruded), and the rod is then cutinto smaller preform sections like that shown in FIG. 1 for receipt bythe insert 10 of FIG. 2. A first end of the rivet preform 1 projectsupwardly into a female cavity 14 at the top of insert 10. A male die 16which cooperates with a conventional press (not shown) is supportedabove the female cavity 14 at the top of insert 10 so as to lie inspaced axial alignment with the first end of the rivet preform 1. A plug18 is positioned within the mold base 12 so as to communicate with thebottom of insert 10. The plug 18 includes a pin 20 that projectsupwardly within the insert 10 so as to lie in spaced axial alignmentwith the opposite end of the rivet preform 1.

The steps by which the rivet preform 1 of FIG. 1 is headed in order toproduce the fiber reinforced composite rivet 30 of FIG. 3 are nowdescribed while continuing to refer to FIG. 2. The rivet preform 1 ispressed into the insert 10 so that the first end of preform 1 projectsupwardly into female cavity 14, as shown. The insert 10, the rivetpreform 1 and the male die 16 are all initially preheated to about 780degree F. in a suitable oven. The mold base 12 and the bottom plug 18are heated within the press to a temperature of approximately 500degrees F. The heated preform 1, insert 10, and male die 16 are removedfrom the oven and placed in the preheated mold base 12 within the press.

The press is now closed to apply approximately 1,500 pounds of pressurefor about three minutes to the first end of the heated rivet preform 1by way of the heated male die 16. The corresponding pressure applied bymale die 16 causes the composite material at the first end of preform 1to soften and flow into the female cavity 14 of heated insert 10, suchthat a relatively wide and flat head (designated 32 in FIG. 3) is formedafter cooling. The extension 20 of plug 18 applies holding pressure tothe opposite end of the rivet preform 1 to prevent the preform frombeing extruded out of the bottom of the insert 10 during the formationof the head 32. While the head 32 is shown as being flat, other shapesare contemplated depending upon the shape of the female cavity 14 withinwhich the composite material of preform 1 is forced.

Once the headed fiber preform has cooled down and solidified within theinsert 10, the press is opened and the male die 16, mold base 12, insert10, and preform 1 are all removed therefrom and turned upside down.Next, the combination of the male die 16, insert 10 and headed preformare separated from the mold base 12 and cooled in water, or the like, toa temperature preferably below 200 degrees F. The cooled combination isthen placed in a well-known arbor press which pushes the male die 16 andthe headed rivet preform 1 out of the insert 10. At this point, the maledie 16 is simply pulled off and separated from the headed preform.

FIG. 3 of the drawings shows a fiber reinforced composite rivet 30having a head 32 at one end thereof after the male die 16 has beenseparated from the preform 1 and the rivet 30 has been deflashed. Theheaded rivet 30 is characterized by the same continuous andunidirectional (i.e., longitudinally extending) fibers 3 and braidedjacket 5 that were first described when referring to FIG. 1. In thissame regard, FIG. 5 of the drawings more clearly illustrates thecontinuous and unidirectional fiber orientation as well as the braidedjacket of the fiber reinforced composite rivet 30 after the end thereofthat lies opposite the head 32 has been upset in a manner that will nowbe disclosed.

To this end, and turning to FIG. 4 of the drawings, the steps aredescribed by which the opposite end of the fiber reinforced compositerivet 30 of FIG. 3 is upset. As indicated above, the composite rivet 30has particular application for securing opposing composite structuralmembers together. By way of example, FIG. 4A shows the composite rivet30 inserted through a pair of axially aligned holes that are formed in apair of composite plates 34 and 36 that are stacked one above the other.The head 32 at the first end of rivet 30 is positioned at one side ofthe plates 34 and 36, and the upstanding core 38 of rivet 30 extendsthrough the plates 34 and 36 to the opposite side thereof.

The upstanding end of core 38 of composite rivet 30 is upset after beinginserted through plates 34 and 36 by means of a forming die 40 and aforming die guide 50. The forming die 40 and the forming die guide 50are preferably manufactured from heat treated tool steel. The leadingend of forming die 40 includes a generally bowl-shaped forming cavity 44and a central pointed tip 46 projecting outwardly past the formingcavity 44. The pointed tip 46 at the leading end of forming die 40 isimportant for directing the flow of continuous fibers at the upset endof the fiber reinforced composite rivet 30 in a manner to be describedwhile referring to FIG. 5 so as to advantageously maximize the abilityof rivet 30 to withstand tensile loads. The trailing end of forming die40 lying opposite the forming cavity 44 and pointed tip 46 includes arecess 48 extending axially therewithin.

A ram 52 having a guide pin 54 projecting outwardly therefrom is spacedabove the forming die 40 so that the guide pin 54 can be moved towardsand into receipt by the recess 48 that is formed in the trailing end offorming die 40. In this way, the guide pin 54 can be moved to accuratelyposition the forming die 40 relative to the upstanding end of the core38 of the fiber reinforced composite rivet 30 that is to be upset.

FIG. 4B shows the guide pin 54 of ram 52 located within the recess 48 atthe trailing end of forming die 40 so that the pointed tip 46 of formingdie 40 is moved into contact with the upstanding end of core 38 ofcomposite rivet 30. In this case, the forming die guide 50 is laid overthe composite plates 34 and 36 to be connected together such that theupstanding end of core 38 of rivet 30 projects upwardly into acontainment opening 58 that is formed in the forming die guide 50. Inthis same regard, the forming die 40 is positioned by the guide pin 54of ram 52 so as to project downwardly into the containment opening 58and thereby engage the upstanding end of core 38. With each of theforming die 40 and the upstanding end of core 38 of rivet 30 axiallyaligned with one another within the containment opening 58 andsurrounded by the forming die guide 50, the ability of the core 38 tospread outside the containment opening 58 of forming die guide 50 willbe blocked during the formation of an upset head (designated 60 in FIGS.4C and 4D).

The details for forming the upset head 60 at the upstanding end of thecore 38 of the fiber reinforced composite rivet 30 are now describedwhile referring to FIG. 4C. Initially, the forming die 40 is placed intoan oven and heated to about 1200 degrees F. The precise temperature towhich the forming die 40 is heated will depend upon the compositematerial from which the rivet 30 is made. Once the forming die 40 hasbeen heated, it is removed from the oven and coupled to the guide pin 54of ram 52, as shown. With the upstanding end of the core 38 of compositerivet 30 extending through the composite plates 34 and 36 and projectinginto the containment opening 58 in the forming die guide 50, the ram 52pushes the heated forming die 40 downwardly through the containmentopening 58 against the upstanding end of core 38. The ram 52 generatesapproximately 100-200 pounds of pressure for about 15 to 30 seconds tocause the upstanding end of core 38 to soften and flow into the formingcavity 44 (best shown in FIG. 4A) at the leading end of forming die 40,whereby to shape the upset head 60. In the alternative, the upstandingend of the core 38 can be softened by conventional ultrasonic techniquesor other rapid heating techniques, such as induction heating.

It is to be understood that a backing force or pressure (not shown) mustbe applied to the lower composite plate 34 to oppose the pressure thatis generated by the ram 52 and thereby prevent the core 38 of rivet 30from being pushed downwardly and outwardly from the composite plates 34and 36 that are to be connected together. Moreover, the ram 52 ispreferably manufactured from a heat conductive metal (e.g., aluminum) soas to draw heat away from the forming die 40 during the formation of theupset head 60 so as to facilitate a rapid cooling.

After the upset head 60 of composite rivet 30 has cooled and solidifiedunder pressure, the ram 52 is raised and the forming die 40 is liftedoff the upset head. As indicated above, the forming die guide 50surrounds both the upstanding end of core 38 and the forming die 40 toprevent the fibers of the composite rivet from spreading outside thecontainment opening 58 of guide 50 under the pressure that is generatedby the ram 52 during the formation of the upset head 60. Accordingly,and as is best shown in FIG. 4D, a fiber reinforced composite rivet 30-1is produced having a generally bowl shaped upset head 60 formed abovethe upper plate 36 and a flat head 32 that is flush with the lower plate34, whereby to reliably hold the pair of composite plates 34 and 36together. Because the upset rivet 30-1 as well as the plates 34 and 36that are held together by rivet 30-1 are all manufactured from acomposite material, each will have the same or substantially similarcoefficient of thermal expansion. Thus, the upset composite rivet 30-1will be able to completely fill the hole through composite plates 34 and36 during changing thermal conditions so as to establish a more reliableconnection therebetween.

The upset head 60 of the composite rivet 30-1 of FIG. 4D has anindentation 62 that is created by the pointed tip 46 which projects fromthe leading end of forming die 40 into the upstanding end of core 38while the core is softened, shaped and cooled within the forming cavity44. As previously described, the pointed tip 46 directs the flow of theunidirectional (i.e., longitudinally extending) fibers that run throughthe fiber reinforced composite rivet 30-1. More particularly, andreferring now to FIG. 5 of the drawings, the fiber orientation of thecomposite rivet 30-1 is shown after the upset head 60 has been formed.The composite rivet 30-1 is shown in a double flush connection in FIG. 5having the flat head 32 thereof positioned flush with the bottomcomposite plate 34 and the opposite upset head 60 positioned flush withthe lower composite plate 36. However, the rivet 30-1 may also beconnected to plates 34 and 36 in a single flush configuration as shownin FIG. 4D or in other rivet configurations such as, for example, aprotruding head configuration (not shown).

The resin impregnated fibers 3 are shown in FIG. 5 running continuouslyand unidirectionally through the rivet 30-1. The indentation 62 that iscreated in the upset head 60 by the pointed tip 46 of forming tool 40splits the flow and directs or flares the fibers 3 outwardly to theperiphery of the upset head 60 to maximize the strength thereof. Thestrength of the composite rivet 30-1 is further increased by virtue ofthe braided jacket 5 which surrounds the core 38. That is, because ofthe ability of the braided jacket 5 to expand and contract, theorientation of the fibers 3 is more likely to follow the contour of therivet 30-1 to maximize the tensile strength thereof. In addition, duringexpansion, the braided jacket 5 aids in directing the unidirectionalfibers 3 to the periphery of the upset head 60 so as to help achieveoptimal tensile characteristics.

1-10. (canceled)
 11. A method for making a composite rivet having anenlarged rivet head at one end and an upset head at the opposite end forholding structural members together, said method comprising the stepsof: providing a rivet preform including a composite core having aplurality of unidirectional fibers running continuously therethrough;locating said rivet preform in an insert having a forming cavity suchthat a first end of said rivet preform projects into said formingcavity; heating said insert and said rivet preform located therein;applying pressure to the first end of said heated rivet preform forsoftening and shaping said first end within the forming cavity of saidinsert to form said enlarged rivet head; cooling said rivet preform toproduce said composite rivet with said enlarged rivet head having ashape corresponding to the shape of the forming cavity of said insertand then removing said composite rivet from said insert; inserting saidcomposite rivet through the structural members to be held together suchthat the enlarged rivet head of said composite rivet lies at one side ofthe structural members and the upstanding end of said composite rivetopposite the enlarged rivet head projects outwardly from the structuralmembers to the opposite side thereof; and forming said upset head byapplying heat and pressure to the upstanding end of said compositerivet.
 12. The method for making recited in claim 11, including theadditional step of surrounding the composite core of said rivet preformwith a braided jacket comprising fibers arranged in a criss-cross weave.13. The method for making recited in claim 11, including the additionalsteps of locating a forming die within the forming cavity of said insertso as to lie in axial alignment with the first end of said heated rivetpreform; and moving said forming die into contact with said first endfor applying said pressure to said first end for softening and shapingsaid first end within the forming cavity of said insert.
 14. The methodfor making recited in claim 13, including the additional step of heatingsaid forming die before the step of moving said forming die into contactwith the first end of said heated rivet preform for applying saidpressure thereto.
 15. The method for making recited in claim 14,including the additional steps of positioning said heated insert, saidheated rivet preform located in said insert, and said heated forming dielocated within the forming cavity of said insert within a preheated moldbase of a press; and closing said press against said heated forming diefor moving said heated forming die into contact with the first end ofsaid heated rivet preform for applying said pressure thereto.
 16. Themethod for making recited in claim 11, wherein said step of forming saidupset head includes the additional steps of: locating an upsetting dieguide having a containment opening extending therethrough so that theupstanding end of the composite rivet is received in said containmentopening; moving an upsetting die having an upsetting cavity into thecontainment opening of said upsetting die guide for applying saidpressure to the upstanding end of the composite rivet; softening andshaping the upstanding end of the composite rivet within the upsettingcavity of said upsetting die; and cooling the upstanding end of thecomposite rivet to form said upset head having a shape corresponding tothe shape of said upsetting cavity.
 17. The method for making recited inclaim 16, wherein said step of forming said upset head by applying heatto the upstanding end of said composite rivet includes heating saidupsetting die for softening and shaping said upstanding end within theupsetting cavity of said upsetting die.
 18. The method for makingrecited in claim 17, including the additional steps of coupling a ram tosaid heated upsetting die; and moving said ram toward said upsetting dieguide for forcing said heated upsetting die against the upstanding endof the composite rivet and thereby applying said pressure to saidupstanding end when said upstanding end is softened and shaped withinthe upsetting cavity of said heated upsetting die.
 19. The method formaking recited in claim 16, including the additional step of forming anindentation within the upset head of the composite rivet.
 20. The methodfor making recited in claim 19, including the additional step of formingsaid indentation within said upset head when said upsetting die is movedinto the containment opening of said upsetting die guide by means of atip carried by said upsetting die and projecting from within saidupsetting cavity thereof.
 21. The method for making recited in claim 11,wherein the step of forming said upset head includes the additionalsteps of: positioning a forming die guide having a containment openingextending therethrough such that the upstanding end of the compositerivet is located in said containment opening; heating a forming diehaving a forming cavity at one end thereof and a tip projecting fromsaid forming cavity; moving said heated forming die towards said formingdie guide such that the upstanding end of the fiber-reinforced compositerivet is received within the forming cavity of said heated forming dieat the containment opening of said forming die guide so as to soften theupstanding end of the fiber-reinforced composite rivet within saidforming cavity and prevent the softened end from spreading outside thecontainment opening of said forming die guide, the tip projecting fromsaid forming cavity making an indentation in the softened rivet end forcontrolling the direction of the continuous fibers with which the rivetis reinforced; and cooling the upstanding end of the fiber-reinforcedcomposite rivet to form said upset head having a shape corresponding tothe shape of said forming cavity where the continuous fibers flow aroundthe indentation and into the upset head to maximize the strength of thecomposite rivet and said upset head thereof.
 22. The method forupsetting a composite rivet recited in claim 21, including theadditional step of heating said forming die in a furnace for softeningthe upstanding end of the fiber-reinforced composite rivet within theforming cavity of said forming die.
 23. The method for upsetting acomposite rivet recited in claim 21, including the additional step ofapplying pressure to said forming die when the upstanding end of thefiber-reinforced composite rivet is softened within the forming cavityof said forming die.
 24. The method for upsetting a composite rivetrecited in claim 23, including the additional steps of coupling a ram tosaid forming die; and moving said ram towards said forming die guide soas to force said forming die against the upstanding end of thefiber-reinforced composite rivet for applying said pressure to saidforming die when the upstanding end of the composite rivet is softenedwithin the forming cavity of said forming die.
 25. The method forupsetting a composite rivet recited in claim 24, including theadditional step of coupling said ram to said forming die by way of aguide pin projecting outwardly from said ram for receipt by said formingdie.